This project (1) examines cancer incidence and mortality among populations exposed to ionizing radiation, especially at low dose levels; (2) characterizes the risk of radiation-induced cancer in terms of tissues at risk, dose response, radiation quality, fractionation of dose, time since exposure, sex, age at exposure and at observation, and possible modifying influences of other environmental and host factors; and (3) examines, tests, and formulates models of radiation carcinogenesis to help define basic mechanisms. Groups studied include the Japanese A-bomb survivors, and several large populations with documented therapeutic (e.g., cervical cancer patients), diagnostic (e.g., tuberculosis patients), and occupational (e.g., x-ray technologists) exposures to ionizing radiation. Program members serve on committees advising the government as well as international agencies. Results of studies suggest that (1) susceptibility to radiogenic breast cancer declines with increasing age at exposure, and children exposed under age 10 are at high risk; a risk at 8-16s rad has been detected; (2) repeated exposure to relatively low radiation doses poses some future risk of breast and thyroid cancer, but not lung cancer; (3) children irradiated for benign conditions of the head and neck are at risk of devloping thyroid and brain neoplasia; (4) 9% of all thyroid cancers may be attributed to prior childhood irradiation; (5) radiotherapy for childhood cancer was associated with subsequent cancers of the bone, connective tissue and thyroid, but not leukemia; (6) actinomycin-D does not appear to protect against radiation-induced thyroid cancer; (7) high-dose radiation to the pelvis induces fewer leukemias than other types of exposures; cell-killing appears to play an important role in defining dose-response relationships; (8) radiation of the adrenal glands may lower breast cancer risk; (9) chromosome aberrations following partial-body irradiation persist in circulating lymphocytes for over 30 years.